Ereditarietà delle strutture della cromatina

Cells maintain identity through epigenetically inherited chromatin structures and histone modifications that are passed from parent to daughter cells. These features—including specific packaging patterns, centromere organization, and histone modifications—create distinct gene expression patterns without changing the underlying DNA sequence. This allows liver cells to produce only liver cells and skin cells to produce skin cells through successive divisions.

CENP-A is a histone H3 variant that initiates centromere formation by binding to AT-rich satellite DNA to create centromere-specific nucleosomes. Once established, CENP-A cooperatively recruits additional CENP-A histones to maintain the centromere structure. This histone variant ensures that centromere organization and function remain stably inherited through cell divisions independent of DNA sequence.

During DNA replication, the histone octamer breaks down into H2A-H2B dimers and H3-H4 tetramers. The H2A-H2B dimers are completely removed, while H3-H4 tetramers are loosely attached and randomly distributed to daughter strands. Newly synthesized H3-H4 tetramers fill remaining spaces, followed by addition of both original and new H2A-H2B dimers to complete the octamer.

Acetylation of histone H3 on newly synthesized DNA marks euchromatin regions, while deacetylation establishes heterochromatin domains. Histone H3 methylation promotes chromatin condensation and heterochromatin formation. After replication, methylated histones are randomly distributed to daughter strands and then associate with histone methyltransferase to methylate newly synthesized histone H3 octamers.

Female mammals inactivate one X-chromosome through dosage compensation, initiated by the long non-coding RNA XIST binding along the chromosome's entire length during embryonic development. Once inactivated, the chromosome remains in this inactive state through successive cell divisions in all somatic cells. This demonstrates how epigenetic modifications, rather than DNA sequence changes, are stably inherited to maintain cellular phenotypes.

The histone code consists of chemical modifications to histones—including methylation, acetylation, and phosphorylation—that serve as signals for gene regulation. These modifications, along with histone variants, constitute the primary carriers of epigenetic marks. The histone code enables cells to maintain differential gene expression patterns and tissue-specific phenotypes through cell divisions without altering DNA sequences.

PCNA is a DNA processivity factor that links the replication machinery to the inheritance of epigenetic marks. At the replication fork, PCNA coordinates nucleosome displacement, histone distribution, and the deposition of both parental and newly synthesized histones on daughter strands. This connection ensures that histone modifications and chromatin organization patterns are faithfully transmitted during DNA replication.